Well, look in your mechanics of materials text for sample of the graph form you're after and then think of how to implement that in Matlab. There isn't, as far as I know, a specific function prepackaged altho if it's allowed by your homework ground rules, you could check the File Exchange for a submission--might have been something done there. Or, I suppose one of the mechanically-oriented toolboxen might have some specialty graphs; those I "know nuthink'" about...
help graph2d % get you started on the various types of plots in Matlab
Big Hint:
do stairs % may be of interest particularly for the shear force diagram
